Algorithms and sequences

Learn

What you’ll learn

• What an algorithm is
• Why robots need precise, ordered instructions
• The three building blocks: sequence, selection and repetition

Algorithms

An algorithm is a precise list of steps that solves a problem. Recipes and directions are algorithms. A robot follows its algorithm exactly, with no common sense, so every step must be clear and in the right order.

“Make a sandwich” is vague. “Take two slices of bread. Spread butter on one side of each slice…” is an algorithm.

The building blocks

Block	Means	Example
Sequence	Do steps in order	Drive forward, then turn, then stop
Selection	Choose based on a condition	If the path is blocked, turn
Repetition	Repeat steps	Do this 4 times; or keep going until the line ends

Almost every robot program is these three blocks combined.

Good algorithms are

• Precise: “turn right 90°”, not “turn a bit”.
• Ordered: steps in the right sequence.
• Finite: they end, or repeat on purpose.
• Tested: you walk through them before trusting them.

Examples

Example: drive a square

repeat 4 times:
    drive forward 50 cm
    turn right 90°
stop

Example: the “human robot” game

One person is the robot and only follows exact commands: “step forward 1 step”, “turn right 90°”. Another writes an algorithm to get them to the door. Missing or vague steps show up immediately: the “robot” walks into a chair.

Example: the same idea in robot code

// sequence by time: 0–1 s forward, 1–1.6 s turn, then stop
function loop() {
  const t = millis();
  if (t < 1000) motors(50, 50);
  else if (t < 1600) motors(50, -50);
  else motors(0, 0);
}

Practice

Try it

Sequences

▶ Run↺ Reset

Your code (JavaScript)// Drive forward 1 s, turn ~90°, drive forward 1 s, stop function loop() { const t = millis(); if (t < 1000) motors(50, 50); else if (t < 1600) motors(50, -50); else if (t < 2600) motors(50, 50); else motors(0, 0); }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Write a morning algorithmEasy

Write an 8-step algorithm for getting ready for school. Swap with a friend: can they find a missing or vague step?

Zig-zagMedium

Make the robot drive in a zig-zag: forward, left turn, forward, right turn, repeating. Use millis() % 3200 to repeat a 3.2-second pattern.

Show an answer

function loop() {
  const t = millis() % 3200;
  if (t < 1000) motors(50, 50);
  else if (t < 1600) motors(-50, 50);   // left
  else if (t < 2600) motors(50, 50);
  else motors(50, -50);                 // right
}

Find the bugHard

This algorithm should get a robot through a door: “turn right 90°; drive forward until the wall; drive through the door”. What’s missing? There’s no step to find the door: after reaching the wall, the robot needs to turn and drive along it until there’s a gap, then turn through it. The last step is also vague: how far?

Quick check

1. What is an algorithm?

A type of robotA precise list of steps that solves a problemA sensorA programming language

2. "If the path is blocked, turn" is an example of…

SequenceSelectionRepetitionA variable

3. Which instruction is precise enough for a robot?

Go over thereTurn a bitDrive forward 50 cmBe careful

Check my answers